This invention relates generally to a method for the electrodeposition of the hard metals, osmium, ruthenium or alloys of the same, as a layer on a wear surface. Such hard metals are particularly well suited for coating the working surface of a wire bonding tool of the capillary or wedge type for use in the semiconductor industry. The invention also relates to the deposition of a silicone layer onto the same working surfaces.
Bonding tools of the foregoing type are set forth in U.S. Pat. No. 3,627,192, including a shank of material having a high modulus of elasticity such as tungsten carbide with one end portion forming a bonding tip. A wire guiding hole presents the wire to be bonded to the tip. The wear surface of the end portion of the tool is formed of osmium alloy which is brazed to the tungsten carbide shank. Osmium is an extremely hard material. It provides an improved wire bond and increased tool life. Bonding wedges and capillaries of various shapes and designs are known.
Separate osmium tips of the foregoing type are subject to a number of disadvantages. Such tips are difficult to braze precisely to the tungsten carbide shank leading to a rate of rejects on the order of 10-20%. Furthermore, in economical practice the wire opening is formed primarily of tungsten carbide with a substantially higher coefficient of friction than osmium, leading to wear out and roughening of the barrel on movement of the wire in the opening. Also, each tip is formed individually with precision brazing, grinding and shaping which is inefficient and costly on a mass production scale.
In U.S. Pat. No. 3,622,474, a technique is disclosed for electrodepositing osmium onto low-emission grid wires in cathode ray tubes to utilize its property of low electron emissivity not its wear resistance or low coefficient of friction. The osmium is disclosed as being plated from an aqueous plating solution at elevated temperatures approaching the boiling point of the water. Thus, there is a severe evaporation problem during electroplating. The disclosed technique would not be effective for plating the surface of microstructural bonding tools of the foregoing type because of the relatively high surface tension of the aqueous electrolytic bath which causes the solution to uncontrollably wet the surface. This results in an uneven coating which distorts the precise microgeometry of the substrate. Also, the grain size of the electrodeposited metal is relatively coarse which adds to further distortion of the microgeometry of the plated surface. This lack of precision from the above two sources is seriously detrimental to the precise coating of microsurfaces such as extremely fine openings in the bonding tools of the type set forth in the aforementioned U.S. Pat. No. 3,627,192.
Another problem common to use the foregoing bonding tools is galling or depositing of the wire on the working surface of the tools. This typically leads to the requirement of performing at least about a dozen wire test bonds with each tool prior to formation of repeatable good bonds.
Tool steel is an inexpensive material from which to form a bonding tool. However, it is subject to rusting and the above galling problem.